How thorough would the destruction be? Can recovery tools be used to recover logs?
If the destruction was complete, there will be no evidence that pirated files were on the computer even if they did exist or not. Also what about false positives?
You should be able to go to the ISP and get a log of the last IP packets sent to the machine before the destruction of the data and trace the IP back to the sender.
But you will know who is responsible because they are supposed to warn you first before they do something like that.